1. Field of the Invention
The present invention relates in general to electronic circuitries and, more particularly, to methods and apparatuses for recognizing a valid change-of-state of capacitively coupled signals using adaptively active impedance.
2. Description of the Related Art
It is common practice in industry to establish communication from a plurality of input contacts 2 of an electronic circuitry in one device to a plurality of output contacts 4 of another by connecting the corresponding contacts from each via a cable bundle 6, as illustrated in FIG. 1. One example of such is the connection of breaker contact outputs with a protection relay contact inputs in a substation of a power grid. When a breaker contact closes, a voltage is applied to the contact input. If that voltage is maintained above a preset threshold value for a predetermined amount of time (typically known as the debounce timer), the contact input recognizes that the applied voltage corresponds to a valid state change and thus the state of that input contact is changed from an OFF state to an ON state. The process is similar for an input transition from the ON state to the OFF state when the applied voltage drops below a second preset threshold for the duration of the debounce timer.
However, the long bundle of cables typically used to connect these devices is known to introduce large parasitic capacitive coupling between individual conductors carrying the monitored signals. This is illustrated in FIG. 2, where a simplified circuit is illustrated to represent the assembly of FIG. 1. In FIG. 2, the contact inputs 2 have respective impedance Ra and Rb and switches SWa and SWb represent the contact outputs 4. In operation, if switch SWa closes, a valid signal will appear continuously on the corresponding contact input Ra. However, a capacitively coupled signal will also discharge via a parasitic capacitance (Cline, as shown in FIG. 2) through the contact input Rb (shown by the dotted line in FIG. 2), even though the switch SWb is open.
Two conventional solutions exist for the above-noted problem. The first includes increasing the debounce timer, and the second includes lowering of the input impedance. In order to reject the capacitively coupled pulse, a validation timer (also known as a debounce timer) can be used to ignore all pulses under a preset duration. The user must configure the debounce timer to ignore pulses with durations less than the worst-case capacitively coupled pulse. However, one of the drawbacks in increasing the duration of the debounce timer is that it delays the recognition of valid contact input transitions, which in turn can affect the efficiency of the protection scheme. Another approach used to mitigate the effects of capacitively coupled transients is to reduce the impedance of the contact input. By reducing this impedance the capacitively coupled signal has a shorter pulse duration that can then allow for a smaller debounce timer setting. However, another drawback of this solution is that the amount of power dissipated by the contact input circuitry increases as the impedance decreases, leading to a limitation in the number of contact inputs that may be available in the product. Therefore in order to prevent false input state changes due to capacitively coupled transients either the number of contact inputs must be reduced or the recognition time for an input state change must be increased.
Conventional solutions to the above-summarized challenge have involved finding a balance between response time (i.e., the time before the contact input can successfully determine if the signal was a transient pulse or a valid input state change) and power dissipation (i.e., how many contact inputs can be used in a device without destroying the circuitry through heat dissipation). However, this balance between power dissipation and contact input response time leads to the current practical limitations within the industry for the number of inputs that can be designed into a product as well as the practical limitations to the response time of a contact input to a valid signal transition
Therefore, a need exist to control the impedance of a contact input such that the current consumed thereby is increased only during the period of time when the input is either in the OFF state or in a transition state and reduced during the period of time when the contact input is in the ON state, in which power consumption is at its peak. Such an approach will not only allow the contact input to consume substantially less power during steady state operation than conventional contact inputs, but will also allow for a significant improvement in recognition time. Since the contact input is only drawing an increased amount of current for a short duration, the amount of current drawn can be maximized for recognition time performance.